Multi-shell structures and fabrication methods for battery active materials with expansion properties

ABSTRACT

Battery electrode compositions are provided comprising core-shell composites. Each of the composites may comprise, for example, an active material, a collapsible core, and a shell. The active material may be provided to store and release metal ions during battery operation, whereby the storing and releasing of the metal ions causes a substantial change in volume of the active material. The collapsible core may be disposed in combination with the active material to accommodate the changes in volume. The shell may at least partially encase the active material and the core, the shell being formed from a material that is substantially permeable to the metal ions stored and released by the active material.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent claims priority to ProvisionalApplication No. 61/661,336 entitled “Multi Shell Structures Designed forBattery Active Materials with Expansion Properties” filed on Jun. 18,2012, which is expressly incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure relates generally to energy storage devices, andmore particularly to metal-ion battery technology and the like.

2. Background

Owing in part to their relatively high energy densities, light weight,and potential for long lifetimes, advanced metal-ion batteries such aslithium-ion (Li-ion) batteries are desirable for a wide range ofconsumer electronics. However, despite their increasing commercialprevalence, further development of these batteries is needed,particularly for potential applications in low- or zero-emissionhybrid-electrical or fully-electrical vehicles, consumer electronics,energy-efficient cargo ships and locomotives, aerospace applications,and power grids.

Accordingly, there remains a need for improved batteries, components,and other related materials and manufacturing processes.

SUMMARY

Embodiments disclosed herein address the above stated needs by providingimproved battery components, improved batteries made therefrom, andmethods of making and using the same.

According to various embodiments, various battery electrode compositionsare provided comprising core-shell composites. Each of the compositesmay comprise, for example, an active material, a collapsible core, and ashell. The active material may be provided to store and release metalions during battery operation, whereby the storing and releasing of themetal ions causes a substantial change in volume of the active material.The collapsible core may be disposed in combination with the activematerial to accommodate the changes in volume. The shell may at leastpartially encase the active material and the core, the shell beingformed from a material that is substantially permeable to the metal ionsstored and released by the active material.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIG. 1 illustrates an example battery electrode composition comprisingcore-shell composites according to certain example embodiments.

FIG. 2 illustrates an alternative example core-shell composite designaccording to other example embodiments.

FIG. 3 illustrates a particular example core-shell composite designutilizing a curved linear backbone according to other exampleembodiments.

FIG. 4 illustrates a particular example core-shell composite designutilizing a curved planar backbone according to other exampleembodiments.

FIGS. 5-6 illustrate two example core-shell composite designs utilizinga porous substrate in combination with a porous filler according toother example embodiments.

FIG. 7 illustrates a particular example core-shell composite designhaving a central void according to other example embodiments.

FIG. 8 illustrates a particular example core-shell composite designhaving a larger central void according to other example embodiments.

FIG. 9 illustrates a particular example core-shell composite designwhere the shell includes a protective coating according to certainexample embodiments.

FIG. 10 illustrates a particular example core-shell composite designwhere the shell includes a porous coating according to certain exampleembodiments.

FIGS. 11-14 are cutaway views of a portion of different example porouscoatings for use as a shell in various embodiments.

FIGS. 15-17 illustrate three particular example core-shell compositedesigns where the shell is a composite material according to variousembodiments.

FIGS. 18-21 illustrate four example core-shell composite designsutilizing discrete particles of the active material according to variousembodiments.

FIG. 22 illustrates a still further example core-shell composite designhaving an irregular shape according to other embodiments.

FIG. 23 illustrates an electrode composition formed from agglomeratedcore-shell composites according to certain embodiments.

FIGS. 24-25 illustrate still further example composite designs accordingto other embodiments.

FIGS. 26A-26E provide experimental images showing various phases offormation for a particular example embodiment.

FIG. 27 provides electrochemical performance data of an example anodecomposite containing high surface area silicon nanoparticles.

FIG. 28 illustrates an example battery (e.g., Li-ion) in which thecomponents, materials, methods, and other techniques described herein,or combinations thereof, may be applied according to variousembodiments.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

The present disclosure provides for the use and formation of activecore-shell composites designed to accommodate volume changes experiencedby certain active materials during battery operation, in which theinsertion and extraction of metal ions may cause the active material tosignificantly expand and contract. According to various embodimentsdescribed in more detail below, a “collapsible” core is provided incombination with the active material and one or more shell layers thatmay be variously deployed for different purposes. The collapsible coreinside the composite structure provides space for expansion of theactive material during insertion of the ions (e.g. metal ions, such asLi ions) during the battery operation. The shell may be variouslyconstructed of different layers to provide, for example, protection ofthe surface of the active material from undesirable reactions with airor with a binder solvent used in electrode formation, to provide furthervolume accommodations for expansion/contraction of the active material,to provide an outer (rigid) structure relatively permeable to the metalions but, in some cases, relatively impermeable to electrolytesolvent(s) in order to have a smaller electrode surface area in directcontact with the electrolyte, and to provide other advantages describedin more detail below. Reduction in the electrode/electrolyte interfacialarea allows for fewer undesirable reactions during battery operation.For example, in cases where the core-shell composite particles are usedin an anode of a metal-ion battery with an organic solvent-basedelectrolyte operating in a potential range, when the electrolyteundergoes a reduction process with the solid electrolyte interphase(SEI) formation, preventing electrolyte solvent transport into the coreby making a shell largely impermeable to the solvent reduces the totalSEI content and irreversible electrolyte and metal ion consumption.Furthermore, by reducing or largely preventing the core-shell compositeparticles from changing their outer dimensions, a significantly morestable SEI layer can be established. Composites of this type have beenshown to exhibit high gravimetric capacity (e.g., in excess of about 400mAh/g for anodes and in excess of about 200 mAh/g for cathodes) whileproviding enhanced structural and electrochemical stability.

FIG. 1 illustrates an example battery electrode composition comprisingcore-shell composites according to certain example embodiments. Asshown, each of the composites 100 includes an active material 102, acollapsible core 104, and a shell 106. The active material 102 isprovided to store and release metal ions during battery operation. Asdiscussed above, for certain active materials of interest (e.g.,silicon), the storing and releasing of these metal ions (e.g., Li ionsin a Li-ion battery) causes a substantial change in volume of the activematerial, which, in conventional designs, may lead to irreversiblemechanical damage, and ultimately a loss of contact between theindividual electrode particles or the electrode and underlying currentcollector. Moreover, it may lead to continuous growth of the SEI aroundsuch volume-changing particles. The SEI growth, in turn, consumes metalions and reduces cell capacity. In the design shown here, however, thecollapsible core 104 is disposed in combination with the active material102 to accommodate such changes in volume by allowing the activematerial 102 to expand inward into the collapsible core 104 itself,rather than expanding outward. The shell 106 at least partially encasesboth the active material 102 and the core 104. The shell 106 may beformed from various layers but in general includes a material that issubstantially permeable to the metal ions stored and released by theactive material, so as not to impede battery operation.

In some embodiments, the collapsible core 104 may be formed from aporous material that absorbs the changes in volume via a plurality ofopen or closed pores. In general, the porosity may be between about 20%and about 99.999% void space by volume, or more preferably, betweenabout 50% and about 95% void space. In the design of FIG. 1, the poresmay be kept small enough to keep the active material 102 from depositinginside the core 104 during synthesis, and instead deposit on the outsideof the core 104 as shown. In some embodiments, the porous material ofthe core 104 may also be electrically-conductive to enhance electricalconductivity of the active material 102 during battery operation. Anexample porous material is a carbon sphere made from carbonized polymerprecursors which is then activated (e.g., by exposure to an oxygencontaining environment such as CO₂ gas or H₂O vapors at elevatedtemperatures of around 500-1100° C.) to remove about 50% to about 95% ofthe material in, preferably, sub-3 nm pores. The porous material mayalso advantageously be electrochemically inert in the battery, such as aporous polymer having no reduction-oxidation reactions in the potentialrange where ions are inserted or extracted from the electrode, thoughmaterials such as carbon (generally not inert if used as an anode in aLi-ion cell, for example) may also be advantageous.

Various methods may be utilized to produce core-shell composites such asthe one shown in FIG. 1. For example, one method for the production of asilicon-based active material with a central carbon-based collapsiblecore and carbon-based shell includes the following steps: (a) synthesizemono dispersed polymer particles (e.g., using a polyDVB monomer); (b)oxidize the particles (e.g., at approximately 250° C., for around 8hours); (c) carbonize the particles to form solid carbon spheres (e.g.,at around 900° C. and 10 torr, for around 1 hour); (d) activate thecarbon spheres to remove most of the mass and leave a highly porous(e.g., greater than about 90% pores) core structure behind (e.g., ataround 1015° C., for around 12 hours), having pores that are generallysmall (e.g., less than around 3 nm); (e) deposit the silicon (in thisexample) active material onto the porous cores via thermal decompositionfrom silane (SiH₄) (e.g., at approximately 525° C. in Ar at 1 torr, foraround 1 hour); and (f) deposit a shell, such as a protective carboncoating (discussed in more detail below) via thermal decomposition froma carbon precursor (e.g., at approximately 900° C. in C₂H₄ at 10 torr,for about 5 hours). It may be beneficial to mix the particles betweensteps to reduce agglomeration during deposition.

In the design of FIG. 1, the active material 102 is shown as at leastpartially encasing the porous material of the core 104. With a highlyporous, but solid core, deposition of the active material 102 as acoating around the core 104 is relatively straightforward and the impactof any defects that may be introduced during fabrication is relativelyminimal. However, in other embodiments, the relationship between theactive material 102 and the core 104 may be modified to achieve otheradvantages for a given application.

FIG. 2 illustrates an alternative example core-shell composite designaccording to other example embodiments. In this design, the composite200 is formed such that the active material 102 is interspersed with theporous material of the core 104. Here, the porous material should beionically conductive and electrically conductive. The advantage of thisdesign is that smaller stresses are induced in the shell 106 becausemuch of the stress caused during the expansion of the active material102 is dissipated by the core 104. As a result, the outer shell 106 canbe made thinner but nonetheless remain functional (and largelydefect-free) during battery operation. In addition, higher interfacialarea between the active material 102 and the core 104 helps to retaingood ionic and electrical transport within the core-shell compositeduring battery operation.

In such designs, the porous material of the collapsible core 104 may beprovided not only as an amorphous structure but also include a poroussubstrate formed of one or more curved linear or planar backbones, forexamples.

FIG. 3 illustrates a particular example core-shell composite designutilizing a curved linear backbone according to other exampleembodiments. In this design, the composite 300 is formed from acollection of curved linear backbones 304 serving as the collapsiblecore and providing a substrate for the active material 102. The curvedlinear backbones 304 may comprise, for example, porous carbon strandswith large pores, the surface of which can be coated with the activematerial 102. The curved nature of the linear backbones 304 alsointroduces an element of porosity into the design. It may beadvantageous for this backbone to be electrically conductive andionically conductive. In some designs, the linear building blocks of thelinear backbone 304 can be composed of linked nanoparticles. Advantagesof the linear backbone 304 include its open structure, which makes iteasy for this structure to be coated uniformly by an active materialusing, for example, vapor deposition or electroless deposition methods.This is because the diffusion of the precursor for the active materialwithin the open framework structure of the curved linear backbone isfast. In addition, after this deposition, the active material coatedlinear backbone can remain sufficiently flexible and robust and thuswithstand mixing, calendaring, and various handling procedures withoutfailure.

FIG. 4 illustrates a particular example core-shell composite designutilizing a curved planar backbone according to other exampleembodiments. In this design, the composite 400 is formed from acollection of curved planar backbones 404 serving as the collapsiblecore and providing a substrate for the active material 102. The curvedplanar backbones 404 may comprise, for example, carbon (nano) flakessuch as exfoliated graphite or multi-layered graphene, the surface ofwhich can be coated with the active material 102. The curved nature ofthe planar backbones 304 also introduces an element of porosity into thedesign. One advantage of the planar backbone is its optimal use of thepore space available to accommodate the volume changes in activematerial. In addition, the curved planar morphology may provide highstructural integrity to both the core and the overall core-shellcomposite. Further, the curved planar morphology makes it easy todeposit a conformal shell 106 encasing the composite particles.

In each of these designs, the different substrates may be combined witha porous filler material to further enhance the overall porosity of thecollapsible core. The porous filler material may be similar to thatdiscussed above in conjunction with the design of FIG. 1, leading to acomposite or hybrid design.

FIGS. 5-6 illustrate two example core-shell composite designs utilizinga porous substrate in combination with a porous filler according toother example embodiments. The first example composite 500 in FIG. 5 issimilar to the design of FIG. 3 in which the porous substrate includes acollection of curved linear backbones 304. Here, the composite 500further includes a porous filler 508 interspersed with the curved linearbackbones 304 deployed as the porous substrate. The second examplecomposite 600 in FIG. 6 is similar to the design of FIG. 4 in which theporous substrate includes a collection of curved planar backbones 404.Here, the composite 600 further includes a porous filler 608interspersed with the curved planar backbones 404 deployed as the poroussubstrate. The material used for the filler 508, 608 should ideally beelectrically and ionically conductive. In some designs, it may also beadvantageous to have a strong, electrically and ionically conductiveinterface between both the active material 102 and the filler 508, 608,as well as between the shell 106 and the filler 508, 608. In this case,the battery operation would be more reliable and higher powerperformance would be achieved. In order to reduce the interfacialresistance between the active material 102 and the filler 508, 608, theactive material 102 can be coated with a thin interfacial layer.Conductive carbon is an example of such a layer, which may improve, forexample, electrical conductivity of this interface in some designs.

Returning to FIG. 1, in some designs, the collapsible core 104 may beformed in such a way so as to form a substantial void in the center ofeach composite that provides additional accommodation for changes involume of the active material 102.

FIG. 7 illustrates a particular example core-shell composite designhaving a central void according to other example embodiments. As shown,the composite 700 is formed such that the collapsible core 104 includesa central void 710 that is encased (at least indirectly) by the activematerial 102. One way in which the central void 710 may be formed, forexample, is by polymerizing two different monomers, such as polystyreneand polyDVB. First, a solid polymer core may be created frompolystyrene, followed by a polymer shell created from polyDVB. Asubsequent carbonization process may be used to remove the polystyrenecore (with little or no residual material) while creating a carbonresidual from the polyDVB to form a shell with a hollow center. Thisstructure may then be left as is (as a solid) or activated to removeadditional material until a desired thickness is reached.

In some applications, it may be advantageous for the thickness of anysubstantive material of the collapsible core 104 to be made relativelythin in relation to the central void 710. For example, it may be made nothicker than is needed to stay intact during further processing.Alternatively, the substantive material of the collapsible core 104 maybe removed altogether or nearly altogether such that the central void710 directly contacts the active material 102 at one or more points.

FIG. 8 illustrates a particular example core-shell composite designhaving a larger central void according to other example embodiments. Inthis design, the composite 800 is formed such that the collapsible core104 includes a larger central void 810 that is encased (at leastindirectly) by the active material 102 and formed large enough withinthe collapsible core 104 so as to contact the active material 102 at oneor more points.

Returning again to FIG. 1, the shell 106 may be formed in a variety ofways and include a variety of layers each specially designed to providecorresponding functionality. For example, the shell 106 may include aprotective coating at least partially encasing the active material 102and the core 104 to prevent oxidation of the active material 102. Theshell 106 may also include a porous coating at least partially encasingthe active material 102 and the core 104 to further accommodate changesin volume, within or among the composites.

FIG. 9 illustrates a particular example core-shell composite designwhere the shell includes a protective coating according to certainexample embodiments. Here, the composite 900 includes an active material102, a collapsible core 104, and a protective coating 906, serving asthe shell 106 in the more generic design of FIG. 1. As shown, theprotective coating 906 at least partially encases the active material102 and the core 104. It will be appreciated that the active material102 and the core 104 are shown for illustration purposes as in the moregeneric design of FIG. 1, but may be implemented according to any of thevarious embodiments disclosed herein.

The protective coating 906 may be provided, for example, to preventoxidation of the active material 102. In some applications, it may beparticularly important to avoid oxidation of the surface of the activematerial 102 after its synthesis. One such application is in cases wherea thin (e.g., 1-2 nm) surface layer comprises a substantial amount(e.g., more than about 10%) of the total volume of active material. Forexample, small nanoparticles of silicon with a diameter of 3 nm havenearly 90% of their volume within a 1 nm surface layer. Thereforeexposure of the 3 nm silicon particles to air and the resultingformation of a native oxide would result in a nearly complete oxidation.Deposition and use of the protective coating 906 on the surface offreshly synthesized active material before any exposure to air or otheroxidizing media reduces or prevents such an oxidation.

An example method for depositing a carbon-based protective coatingwithout exposure of a synthesized silicon-based active material to airis as follows. The carbon layer can be deposited by chemical vapordeposition of carbon from one of various hydrocarbon precursors, such asacetylene and propylene, to name a few. In one embodiment, thedeposition may be conducted in the same reactor where silicon depositionor formation is performed. In another embodiment, the chamber wheresilicon is deposited may be subsequently filled with an inert gas (suchas argon or helium) and sealed with valves. To minimize leaks in thesystem, a positive (above atmospheric) pressure may be applied. Thesealed chamber may then be transferred into a carbon deposition tool.The chamber may be connected to the gas lines of the carbon depositiontool, while remaining sealed. Prior to opening the valve connecting thesilicon-containing chamber and the carbon-deposition tool gas lines, theline to the carbon precursor may be evacuated and filled with either aninert gas or a hydrocarbon gas in such a way so as to minimize thecontent of water or oxygen molecules within the system that are to beexposed to silicon during the carbon deposition process. In anotherembodiment, the particles can be transferred internally between siliconand carbon deposition zones using gravity or other powder transfermeans.

It may be advantageous to have the total number of oxygen atoms in thesystem be at least twenty times smaller than the total number of siliconatoms in the silicon nanopowder or silicon nanostructures containedwithin the chamber and to be protected from oxidation by the carbonlayer. In one example, the silicon-containing chamber filled with aninert gas may be heated to an elevated temperature of between about500-900° C. After the desired temperature is reached, the carbonprecursor gas (vapor) may be introduced into the system, depositing acarbon layer onto the silicon surface. In some embodiments, it may beadvantageous to perform carbon deposition in sub-atmospheric pressures(for example, at about 0.01-300 torr) in order to form a morecrystalline, conformal layer with better protective properties. Afterthe deposition of the protective carbon, the chamber can be cooled downto below 300° C., or preferably below 60° C., prior to exposure to air.For this carbon layer to serve as an effective protective barrieragainst oxidation, the thickness of the conformal carbon layer shouldmeet or exceed approximately 1 nm.

FIG. 10 illustrates a particular example core-shell composite designwhere the shell includes a porous coating according to certain exampleembodiments. Here, the composite 1000 includes an active material 102, acollapsible core 104, and a protective coating 1006, serving as theshell 106 in the more generic design of FIG. 1. As shown, the protectivecoating 1006 at least partially encases the active material 102 and thecore 104. Here, the porous coating 1006 is formed with a plurality ofopen or closed pores to further accommodate changes in volume. It willagain be appreciated that the active material 102 and the core 104 areshown for illustration purposes as in the more generic design of FIG. 1,but may be implemented according to any of the various embodimentsdisclosed herein.

In some embodiments, the porous coating 1006 may be composed of a porouselectrically-conductive carbon. An example process for the formation ofa porous carbon layer includes formation of a polymer coating layer andits subsequent carbonization at elevated temperatures (e.g., betweenabout 500-1000° C., but below the thermal stability of the activematerial or the active material's reactivity with the carbon layer).This results in the formation of a carbon containing pores. Additionalpores within the carbon can be formed as desired upon activation undercertain conditions, with the oxidation rate of the active material beingsignificantly lower than the oxidation (activation) rate of porouscarbon. In other embodiments, the porous coating 1006 may comprise apolymer-carbon mixture. In still other embodiments, the porous coating1006 may comprise a polymer electrolyte. Polyethylene oxide (PEO)infiltrated with a Li-ion salt solution is an example of a polymerelectrolyte. If a polymer electrolyte does not have mixed (bothelectronic and ionic) conductivities (as in the case of PEO) but only asignificant ionic conductivity, the porous shell may further comprise anelectrically conductive component, such as carbon, in order to injectelectrons or holes into the active material during battery operation.

As noted above, according to various embodiments, the pores of theporous coating 1006 may be open or closed. In either case, the variouspores may further include different functional fillers, used alone or incombination, as discussed in more detail below.

FIGS. 11-14 are cutaway views of a portion of different example porouscoatings for use as a shell in various embodiments. FIG. 11 illustratesan example design 1100 of the porous coating 1006 shown in FIG. 10 inwhich a plurality of closed pores 1112 are present, at least some of thepores 1112 being filled with a first functional filler material 1114.FIG. 12 illustrates an example design 1200 of the porous coating 1006shown in FIG. 10 in which a plurality of closed pores 1112 are againpresent, and at least some of the pores 1112 are again filled with thefirst functional filler material 1114. However, in this design, at leastsome other pores 1112 are filled with a second functional fillermaterial 1216, creating a composite material of different functionalfillers. FIG. 13 illustrates an example design 1300 of the porouscoating 1006 shown in FIG. 10 in which a plurality of open pores 1318are present and interpenetrating the porous coating 1006. In somedesigns, the open pores 1318 may be formed in combination with theclosed pores 1112, as shown. FIG. 14 is an example design 1400 of theporous coating 1006 shown in FIG. 10 in which a plurality of open pores1318 and closed pores 1112 are present and filled with a givenfunctional filler material 1420.

In some applications, particularly in those where formation of somefraction of small cracks is likely, it is advantageous that at least afraction of the pores within the porous coating be filled withfunctional fillers such as electrolyte additives, which are capable ofsealing the micro-cracks formed within such a layer during metal-ioninsertion into the active particle core and the resulting volumechanges. One example of such an additive is a vinylene carbonate (VC)optionally mixed with a metal-ion (such as Li-ion) containing salt.Another example of such an additive is an initiator for radicalpolymerization, capable of inducing polymerization of the electrolytesolvent(s). Conventional use of these additives (such as VC) has beenlimited to Li-ion battery electrolytes, without any such infiltration orincorporation within a porous layer around the active particles. Thisapproach improves stability of the composite electrodes withoutsignificantly sacrificing other advantageous properties of the bulkelectrolyte. In addition, it allows one to use different additiveswithin porous layers on the surfaces of anodes and cathodes.

In some designs, the shell may be a composite material comprising atleast an inner layer and an outer layer, with potentially one or moreother layers as well. The shell may accordingly be made by combiningdifferent coatings of the types described above and the different layersmay be provided for different functions. For example, one component ofthe shell may provide better structural strength, and another one betterionic conductivity. In another example, one component can provide betterionic conductivity, and another one better electrical conductivity. Insome applications, it may be advantageous to have these componentsinterpenetrate each other. In this case, the composite shell may provideboth high ionic and electrical conductivity if one component is moreelectrically conductive and another one more ionically conductive.

FIGS. 15-17 illustrate three particular example core-shell compositedesigns where the shell is a composite material according to variousembodiments. FIG. 15 illustrates an example composite 1500 in which theinner layer of the shell is a protective coating layer 906 of the typedescribed in conjunction with FIG. 9, and the outer layer is a porouscoating layer 1006 of the type described in conjunction with FIG. 10.Conversely, FIG. 16 illustrates an example composite 1600 in which theinner layer of the shell is a porous coating layer 1006 of the typedescribed in conjunction with FIG. 10, and the outer layer is aprotective coating layer 906 of the type described in conjunction withFIG. 9. The outer protective coating layer 906 in FIG. 16 may offerother useful functionalities. For example, it may prevent electrolytesolvent transport into the porous component of the shell and the core,which reduces the sites of undesirable reactions between electrolyte andthe composite core-shell electrode particles. Formation of an SEI on acore-shell anode operating in the potential range of 0-1.2V vs. Li/Li+in Li-ion batteries is an example of such reactions. This outer coatinglayer 906 (if made impermeable to electrolyte solvent) reduces the totalSEI content and irreversible electrolyte and metal ion consumption.Alternatively, the outer coating layer 906 in FIG. 16 may offer improvedelectrical conductivity, which may enhance capacity utilization andpower characteristics of the electrodes based on the describedcore-shell particles. Further, the outer coating layer 906 in FIG. 16may provide structural integrity to the core-shell particles with avolume-changing active material.

FIG. 17 illustrates an example composite 1700 that further includes anadditional coating layer 1722 at least partially encasing the otherlayers. The additional coating layer 1722 may be formed, for example,from a material that is (i) substantially electrically conductive and(ii) substantially impermeable to electrolyte solvent molecules. In eachillustration, it will again be appreciated that the active material 102and the core 104 are shown for illustration purposes as in the moregeneric design of FIG. 1, but may be implemented according to any of thevarious embodiments disclosed herein.

In some applications, it may be advantageous to provide a solid carbonlayer between porous carbon and silicon. This solid layer may bedeposited in order to prevent the oxidation of the silicon surface, asdiscussed above. In other applications where high surface area pores areopen to the electrolyte and thus available for electrolytedecomposition, it may be advantageous to deposit a solid carbon layeronto the outer surface of the porous carbon layer. This deposition sealsthe pores and reduces the total surface area of the material exposed toelectrolyte. As a result, this deposition reduces undesirable sidereactions, such as electrolyte decomposition. In still otherapplications, both approaches may be used to create a three-layeredstructure.

In addition or alternatively, an additional coating layer may beprovided to impart further mechanical stability. Thus, the outermostshell layer can comprise ion permeable materials other than carbon, suchas metal oxides. In some applications, where minimal volume changes ofthe composites is particularly important, it is advantageous for atleast the outermost shell layer to experience significantly smallervolume changes (e.g., twice as small, or preferably three or more timesas small) than the core active material during battery operation.

A rigid outer shell of this type can be made of carbon or ceramiccoating(s) or both, for example. In one configuration, such a shell canbe made of conductive carbon. The coating can be deposited bydecomposition of carbon containing gases, such as hydrocarbons (theprocess is often called chemical vapor deposition) according to thefollowing reaction: 2C_(x)H_(y)=2xC+yH₂, where C_(x)H_(y) is thehydrocarbon precursor gas. The carbon deposition temperature may be inthe range of about 500-1000° C. After deposition, the core-shellstructure can be annealed at temperatures of about 700-1100° C., butpreferably about 800-1000° C. to induce additional structural orderingwithin the carbon, to desorb undesirable impurities, and to strengthenthe bonding between core and shell.

An alternative method of depositing carbon on the surface of the activematerial includes catalyst-assisted carbonization of organic precursors(e.g., polysaccharide or sucrose carbonization in the presence ofsulfuric acid). Yet another method of producing the carbon coatingincludes hydrothermal carbonization of the organic precursors on thesurface of the active material at elevated temperatures (e.g., about300-500° C.) and elevated pressures (e.g., about 1.01-70 atm). Yetanother method of producing the carbon outer coating includes formationof the polymer around the active material and subsequent carbonizationat elevated temperatures. In addition to a polymer coating, the activematerial can be initially coated with small carbon particles or multi-or single-graphene layers. Carbonization may be used to transform thepolymer-carbon composite outer shell into a conductive carbon-carboncomposite shell.

In addition to pure carbon, a metal-ion permeable shell in this andother described structures may be composed of or contain metal oxides,metal phosphates, metal halides or metal nitrides, including, but notlimited to, the following metals: lithium (Li), aluminum (Al), cobalt(Co), boron (B), zirconium (Zr), titanium (Ti), chromium (Cr), tantalum(Ta), niobium (Nb), zinc (Zn), vanadium (V), iron (Fe), magnesium (Mg),manganese (Mn), copper (Cu), nickel (Ni), and others. They mainrequirements include, but are not limited to, high ionic conductivity incombination with good structural and chemical stability during electrodeoperation in the selected battery chemistry.

Deposition of such coatings can be performed using a variety of oxidecoating deposition techniques, including physical vapor deposition,chemical vapor deposition, magnetron sputtering, atomic layerdeposition, microwave-assisted deposition, wet chemistry, precipitation,solvothermal deposition, hydrothermal deposition, and others incombination with an optional annealing at elevated temperatures (e.g.,greater than about 200° C.). For example, metal oxide precursors in theform of a water-soluble salt may be added to the suspension (in water)of the composites to be coated. The addition of a base (e.g., sodiumhydroxide or amine) causes formation of a metal (Me) hydroxide. Activematerial particles suspended in the mixture may then act as nucleationsites for Me-hydroxide precipitation. Once coated with a shell ofMe-hydroxide, they can be annealed in order to convert the hydroxideshell into a corresponding oxide layer that is then well-adhered totheir surface.

Accordingly, throughout the various embodiments discussed herein, itwill be appreciated that the shell may serve several purposes. First, itmay create a mechanically rigid surface that prevents the activematerial from expanding outwards. Because the core may be highly porousand “soft,” and the active material must expand, the active materialexpands inward, towards the core rather than outwards. Without theshell, the active material might expand inwards and outwards, whichwould cause the outer surface of the structure to change. Second, theshell may also be made ionically conductive for metal ions or the liketo move to the active material. It may also be electrically conductiveso that the composites making up the electrode will make betterelectrical contact with each other. Third, it may advantageously havegood properties for forming SEI in the electrolyte used. Although theexample shell material discussed most prominently above is carbon orcarbon-based, certain oxides and ceramics may also be used to formshells with advantageous properties. Metals may also be used if channelsfor ionic conductivity are formed without compromising the mechanicalintegrity.

Returning again to FIG. 1, the active material 102 may be provided invarious forms according to different embodiments, both for bettermatching a given implantation of the other composite components as wellas for other reasons. In the design of FIG. 1, the active material 102is shown in a generally amorphous or nanocrystalline (grain size below 1micron, preferably below 500 nm) form as conformally coated onto thecollapsible core 104. This amorphous or nanocrystalline form issimilarly shown in FIG. 2 where the active material 102 is interspersedwith the porous material of the core 104, in FIG. 3 where the activematerial 102 is conformally coated onto the curved linear backbones 304,in FIG. 4 where the active material 102 is conformally coated onto thecurved linear backbones 404, and so on. In each of these designs,however, the active material 102 may be provided in an alternative formfor different applications.

FIGS. 18-21 illustrate four example core-shell composite designsutilizing discrete particles of the active material according to variousembodiments. FIG. 18 illustrates a composite 1800 that is similar to thedesign of FIG. 1 but with discrete particles 1802 disposed around thecollapsible core 104. These particles may optionally (but preferably) beelectrically connected to each other and to the shell 106. Theseelectrical connections provide more uniform insertion and extraction ofions from the active material 102. These electrical connections may bedirect (particle-to-particle) or via the collapsible core 104 (whenproduced from an electrically conductive material) or via anelectrically conductive shell 106 (when the shell is electricallyconductive). FIG. 19 illustrates a composite 1900 that is similar to thedesign of FIG. 2 but with discrete particles 1802 interspersed with thecollapsible core 104. FIGS. 20-21 illustrate respective composites 2000and 2100 that are similar to the designs of FIGS. 3-4, respectively, butwith discrete particles 1802 interspersed with their respective cores ontheir different backbone substrates 304, 404.

In any case, the individual particles 1802 may be further coated with aprotective coating to prevent oxidation of the active material. When thediscrete particles 1802 are interspersed with the core 104, they shouldbe electrically connected to each other and to the shell 106. Theseelectrical connections are needed for the reversible electrochemicalreduction and oxidation processes (which take place during normalbattery operation) to proceed. As in the discussion above, theseelectrical connections may be direct (particle-to-particle) or via thecollapsible core 104 (when produced from an electrically conductivematerial) or via electrically conductive links (such as electricallyconductive particles of various shapes maintaining a direct contactbetween the discrete active particles 1802). In the latter twoinstances, there is no requirement for direct contact between thediscrete active particles 1802.

It will be appreciated that these examples are merely provided asexemplary and not an exhaustive list of discrete particle design for theactive material. The other designs disclosed herein for differentarrangements of cores and shells may likewise be implemented usingdiscrete active particles.

In some embodiments, the active material may be a silicon orsilicon-rich material, as in a few of the examples above. In otherembodiments, however, the disclosed techniques may be applied to avariety of higher capacity anode materials including not only silicon,but also other anode materials that experience significant volumechanges (e.g., greater than about 7%) during insertion or extraction oftheir respective metal ions. Examples of such materials include: (i)heavily (and “ultra-heavily”) doped silicon; (ii) group IV elements;(iii) binary silicon alloys (or mixtures) with metals; (iv) ternarysilicon alloys (or mixtures) with metals; and (v) other metals and metalalloys that form alloys with metal ions such as lithium.

Heavily and ultra-heavily doped silicon include silicon doped with ahigh content of Group II elements, such as boron (B), aluminum (Al),gallium (Ga), indium (In), or thallium (Tl), or a high content of GroupV elements, such as nitrogen (N), phosphorus (P), arsenic (As), antimony(Sb), or bismuth (Bi). By “heavily doped” and “ultra-heavily doped,” itwill be understood that the content of doping atoms is typically in therange of 3,000 parts per million (ppm) to 700,000 ppm, or approximately0.3% to 70% of the total composition.

Group IV elements used to form higher capacity anode materials mayinclude Ge, Sn, Pb, and their alloys, mixtures, or composites, with thegeneral formula of Si_(a)—Ge_(b)—Sn_(c)—Pb_(d)—C_(e)-D_(f), where a, b,c, d, e, and f may be zero or non-zero, and where D is a dopant selectedfrom Group III or Group V of the periodic table.

For binary silicon alloys (or mixtures) with metals, the silicon contentmay be in the range of approximately 20% to 99.7%. Examples of such asalloys (or mixtures) include, but are not limited to: Mg—Si, Ca—Si,Sc—Si, Ti—Si, V—Si, Cr—Si, Mn—Si, Fe—Si, Co—Si, Ni—Si, Cu—Si, Zn—Si,Sr—Si, Y—Si, Zr, —Si, Nb—Si, Mo—Si, Tc—Si, Ru—Si, Rh—Si, Pd—Si, Ag—Si,Cd—Si, Ba—Si, Hf—Si, Ta—Si, and W—Si. Such binary alloys may be doped(or heavily doped) with Group III and Group V elements. Alternatively,other Group IV elements may be used instead of silicon to form similaralloys or mixtures with metals. A combination of various Group IVelements may also be used to form such alloys or mixtures with metals.

For ternary silicon alloys (or mixtures) with metals, the siliconcontent may also be in the range of approximately 20% to 99.7%. Suchternary alloys may be doped (or heavily doped) with Group III and GroupV elements. Other Group IV elements may also be used instead of siliconto form such alloys or mixtures with metals. Alternatively, other GroupIV elements may be used instead of silicon to form similar alloys ormixtures with metals. A combination of various Group IV elements mayalso used to form such alloys or mixtures with metals.

Examples of other metals and metal alloys that form alloys with lithiuminclude, but are not limited to, Mg, Al, Ga, In, Ag, Zn, Cd, etc., aswell as various combinations formed from these metals, their oxides,etc.

The disclosed techniques may also be applied to several high capacitycathode active materials, which experience significant (e.g., greaterthan about 7%) volume changes during insertion and extraction of metalions (such as Li ions, for example) during the operation of a metal-ioncell (such as a Li-ion cell).

Examples of high capacity cathode materials include, but are not limitedto, conversion-type cathodes, such as metal fluorides, metaloxy-fluorides, various other metal halides and oxy-halides (such asmetal chlorides, metal bromides, metal iodides) and others. Examples ofmetal fluorides based on a single metal include, but are not limited to,FeF₂ (having a specific capacity of 571 mAh/g in Li-ion batteryapplications), FeF₃ (having a specific capacity of 712 mAh/g in Li-ionbattery applications), MnF₃ (having a specific capacity of 719 mAh/g inLi-ion battery applications), CuF₂ (having a specific capacity of 528mAh/g in Li-ion battery applications), and NiF₂ (having a specificcapacity of 554 mAh/g in Li-ion battery applications). It will beappreciated that metal halides may include two or more different metals.For example, Fe and Mn or Ni and Co or Ni and Mn and Co. The metalhalides mentioned above may also contain lithium (particularly in thecase of Li-ion batteries) or other metals for the correspondingmetal-ion batteries. Finally, metal halide active materials may compriseboth metal atoms in a metallic form and in the form of a metal halide.For example, the metal halide-based active materials may comprise amixture of a pure metal (such as Fe) and a lithium halide (such as LiF)in case of a Li-ion battery (or another metal halide in case of ametal-ion battery, such as sodium halide (such as NaF) in case of aNa-ion battery or magnesium halide (MgF₂) in case of a Mg-ion battery).The pure metal in this example should ideally form an electricallyconnected array of metal species. For example, electrically connectedmetal nanoparticles (such Fe nanoparticles) or electrically connectedcurved metal nanowires or metal dendritic particles or metal nanosheets.Alternatively, the metal-1 component of the active (metal-1/metal-2halide) mixture can form a curved linear or curved planar backbone ontowhich the metal-2 halide is deposited.

The disclosed techniques may also be applied to several high capacityanode and cathode active materials that experience significant volumechanges when used in battery chemistries other than metal-ion batteries.

FIG. 22 illustrates a still further example core-shell composite designhaving an irregular shape according to other embodiments. As shown, thecomposite 2200 is compositionally equivalent to the design of FIG. 1 andincludes an active material 102, a collapsible core 104, and a shell106. It is, however, irregularly shaped to demonstrate that thegenerally spherical shape of various composites illustrated in otherfigures is not required and that other, even irregular shapes arecontemplated.

FIG. 23 illustrates an electrode composition formed from agglomeratedcore-shell composites according to certain embodiments. As shown, eachcomposite of the agglomeration 2300 includes active material particles1802, a collapsible core 104, and a porous shell 1006, similar tovarious design aspects discussed above. In this design, the porousmaterial for the collapsible core 104 and the porous shell 1006 areselected to be the same. Accordingly, as demonstrated in the figure, adesign incorporating such elements effectively blurs the distinctionbetween core and shell, leading to a structure that is equivalent to anagglomeration of composites formed without shells per se (i.e., in thatthe core of one composite acts as a shell for another composite in theagglomeration by providing an equivalent accommodation for volumechanges). Such a design is contemplated herein as well.

FIGS. 24-25 illustrate still further example composite designs accordingto other embodiments. FIG. 24 illustrates a design 2400 including anexample porous active material powder structure 2402 encased in a shell2406 but in which volume changes are accommodated by the porous natureof the active material itself rather than a collapsible core. FIG. 25illustrates a design 2500 including a similar example porous activematerial powder structure 2402 but with a shell 2506 disposed as aconformal coating.

In general, it is noted that composite particles of the type discussedherein can be synthesized from about 50 nm to about 50 μm in size. Thecore and shells can be designed to vary in thickness or diameter fromabout 1 nm to about 20 μm. Electrode designs with a relatively uniformsize distribution of the composites may be beneficial, as propertiesremain consistent from particle to particle. However, it may beadvantageous for other embodiments to create structures of two, three,or more uniform diameters and mix them together to allow for highpacking density when electrodes are fabricated. Because these compositeschange very little if at all in size during cycling on the outersurface, the particle-to-particle connection can stay intact with strongor weak binders.

Composite size is driven by a multitude of factors. In particular,additive CVD processes tend to bind adjacent particles together, forminglarge agglomerates. This is true especially in bulk powder processing.Agglomeration of adjacent composites can be mitigated during bulk powderprocessing in all synthesis processes by any combination of tumbleagitation of the entire powder volume, entrainment of the composites ina fluid flow, dropping composites to maintain separation between them,vibratory agitation, milling, electrostatic charging, or other means.Composite particle size can also be controlled by reducing it aftersynthesis using milling techniques.

FIGS. 26A-26E provide experimental images showing various phases offormation for a particular example embodiment, including (a) polymerizedcore precursor particles (oxidized polyDVB) (FIG. 26A), (b) carbonizedcore particles (FIG. 26B), (c) activated core particles (FIG. 26C), (d)silicon deposited on activated carbon core particles (FIG. 26D), and (e)a carbon shell deposited on silicon on activated carbon core particles(FIG. 26E). It will be appreciated that the example design shown here isfor illustration purposes only, and is not intended to represent theonly or the best implementation.

FIG. 27 provides electrochemical performance data of an example anodecomposite containing high surface area silicon nanoparticles. Dischargecapacity is shown as a function of cycle number and the presence orabsence of a protective carbon layer deposited on the fresh siliconsurface without its exposure to air. The positive impact of theprotective layer on the reversible capacity is evident. Without theprotective coating over 60% of the silicon atoms became oxidized, whichresulted in a significant reduction of the capacity utilization.

FIG. 28 illustrates an example battery (e.g., Li-ion) in which thecomponents, materials, methods, and other techniques described herein,or combinations thereof, may be applied according to variousembodiments. A cylindrical battery is shown here for illustrationpurposes, but other types of arrangements, including prismatic or pouch(laminate-type) batteries, may also be used as desired. The examplebattery 2801 includes a negative anode 2802, a positive cathode 2803, aseparator 2804 interposed between the anode 2802 and the cathode 2803,an electrolyte (not shown) impregnating the separator 2804, a batterycase 2805, and a sealing member 2806 sealing the battery case 2805.

The forgoing description is provided to enable any person skilled in theart to make or use embodiments of the present invention. It will beappreciated, however, that the present invention is not limited to theparticular formulations, process steps, and materials disclosed herein,as various modifications to these embodiments will be readily apparentto those skilled in the art. That is, the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention.

1. A battery electrode composition comprising core-shell composites,each of the composites comprising: an active material provided to storeand release metal ions during battery operation, whereby the storing andreleasing of the metal ions causes a substantial change in volume of theactive material; a collapsible core disposed in combination with theactive material to accommodate the changes in volume; and a shell atleast partially encasing the active material and the core, the shellbeing formed from a material that is substantially permeable to themetal ions stored and released by the active material.
 2. The batteryelectrode composition of claim 1, wherein the collapsible core is formedfrom a porous material that absorbs the changes in volume via aplurality of open or closed pores.
 3. The battery electrode compositionof claim 2, wherein the porous material of the core comprises a porousand electrically-conductive carbon material.
 4. The battery electrodecomposition of claim 2, wherein the active material at least partiallyencases the porous material of the core.
 5. The battery electrodecomposition of claim 2, wherein the active material is interspersed withthe porous material of the core.
 6. The battery electrode composition ofclaim 2, wherein the porous material comprises a porous substrate formedof one or more curved linear or planar backbones.
 7. The batteryelectrode composition of claim 6, wherein the porous material furthercomprises a porous filler interspersed with the porous substrate.
 8. Thebattery electrode composition of claim 1, wherein the collapsible corecomprises a central void that is encased by the active material.
 9. Thebattery electrode composition of claim 8, wherein the central voiddirectly contacts the active material.
 10. The battery electrodecomposition of claim 1, wherein the shell comprises a protective coatingat least partially encasing the active material and the core to preventoxidation of the active material.
 11. The battery electrode compositionof claim 1, wherein the shell comprises a porous coating at leastpartially encasing the active material and the core, the porous coatinghaving a plurality of open or closed pores to further accommodatechanges in volume.
 12. The battery electrode composition of claim 11,wherein at least some of the pores in the porous coating are closedpores filled with a first functional filler material.
 13. The batteryelectrode composition of claim 12, wherein at least some other pores inthe porous coating are closed pores filled with a second functionalfiller material.
 14. The battery electrode composition of claim 11,wherein at least some of the pores in the porous coating are open poresinterpenetrating the porous coating and filled by a functional fillermaterial.
 15. The battery electrode composition of claim 1, wherein theshell is a composite material comprising an inner layer and an outerlayer.
 16. The battery electrode composition of claim 15, wherein theinner layer is one of a protective coating layer or a porous coatinglayer, and wherein the outer layer is the other of the protectivecoating layer or the porous coating layer.
 17. The battery electrodecomposition of claim 16, wherein the inner layer is the protectivecoating layer and the outer layer is the porous coating layer, andwherein the composite material further comprises an additional coatinglayer at least partially encasing the other layers and formed from amaterial that is (i) substantially electrically conductive and (ii)substantially impermeable to electrolyte solvent molecules.
 18. Thebattery electrode composition of claim 1, wherein the active materialcomprises discrete particles disposed around or interspersed with thecollapsible core.
 19. The battery electrode composition of claim 18,wherein the particles are coated with a protective coating to preventoxidation of the active material.
 20. The battery electrode compositionof claim 1, wherein the active material is conformally coated onto thecollapsible core.